The present invention generally relates to a liquid crystal display, and more particularly to a liquid crystal display capable of preventing color shift and having enhanced response time.
A liquid crystal display device has been used in various information display terminals. The major operating system for the liquid crystal display device is the twisted nematic(xe2x80x9cTNxe2x80x9d) and the super twisted nematic (xe2x80x9cSTNxe2x80x9d). Though they are presently commercially used in the market, the problems of narrow viewing angle are still remained unsolved.
An In-Plane Switching (xe2x80x9cIPSxe2x80x9d) mode liquid crystal display has been suggested to solve foregoing problems.
As described in FIG. 1, a plurality of gate bus lines 11 are formed on a lower insulating substrate 10 along an x direction shown in the drawings and the gate bus lines 11 are parallel to each other. A plurality of data bus lines 15 are formed along a y direction which is substantially perpendicular to the x direction.
At this time, a pair of gate bus lines 11 and a pair of data bus lines 15 are formed for defining the sub-pixel region. The gate bus line 11 and the data bus line 15 are insulated by a gate insulating layer(not shown).
A counter electrode 12 is formed, for example in a rectangular frame shape, within a sub-pixel region and it is disposed at the same plane with the gate bus line 11.
A pixel electrode 14 is formed at each sub-pixel region where the counter electrode 12 is formed. The pixel electrode 14 is composed of a web region 14a which divides the region surrounded by the rectangular frame type counter electrode 12 with a y direction, a first flange region 14b connected to a portion of the web region 14a and simultaneously overlapped with the counter electrode 12 of the x direction, and a second flange region 14c which is parallel to the first flange region 14b and is connected to the other portion of the web region 14a. That is to say, the pixel electrode 14 seems to be the letter xe2x80x9cIxe2x80x9d. Herein, the counter electrode 12 and the pixel electrode 14 are made of opaque metal layers.
The pixel electrode 14 and the counter electrode 12 are insulated from each other by a gate insulating layer (not shown).
A thin film transistor 16 (xe2x80x9cTFTxe2x80x9d) is disposed at the intersection of the gate bus line 11 and the data bus line 15. This TFT 16 is composed of a gate electrode being extended from the gate bus line 11, a drain electrode being extended from the data bus line 15, a source electrode being extended from the pixel electrode 14 and a channel layer 17 formed on upper of the gate electrode.
A storage capacitor Cst is disposed at the region where the counter electrode 12 and the pixel electrode 14 are overlapped.
Although not shown in FIG. 1, an upper substrate(not shown) equipped with a color filter(not shown) is disposed with a predetermined distance opposite to a lower substrate 10. Herein, the distance between the upper substrate and the lower substrate 10 is smaller than that between a region of the counter electrode toward the y direction and the web region of the pixel electrode thereby forming a parallel field which is parallel with the substrate surface. Further a liquid crystal layer(not shown) having a plurality of liquid crystal molecules is interposed between the upper substrate (not shown) and the lower substrate 10.
Also, onto the resultant structure of the lower substrate and onto an inner surface of the upper substrate are formed homogeneous alignment layers respectively. By the homogeneous alignment layer, before forming an electric field between the counter electrode 12 and the pixel electrode 14, long axes of liquid crystal molecules 19 are arranged parallel to the surface of the substrate 10. Also, by the rubbing axis of the homogeneous alignment layer, the orientation direction of the molecules 19 is decided. The R direction in the drawings is the direction of rubbing axis for the homogeneous alignment layer formed on the lower substrate 10.
A first polarizing plate(not shown) is formed on the outer surface of the lower substrate 10 and a second polarizing plate(not shown) is formed on the outer surface of the upper substrate(not shown). Herein, the first polarizing plate is disposed to make its polarizing axis to be parallel to the P direction of the FIG. 1. That means, the rubbing axis direction R and the polarizing axis direction P are parallel each other. On the other hand, the polarizing axis of the second polarizing plate is substantially perpendicular to that of the first polarizing plate.
When a scanning signal is applied to the gate bus line 11 and a display signal is applied to the data bus line 15, the TFT 16 disposed at the intersection of the gate bus line 11 and the data bus line 15 is turned on. Then the display signal of the data bus line 15 is transmitted to the pixel electrode 14 through the TFT 16. Consequently, an electric field E is generated between the counter electrode 12 where a common signal is inputted and the pixel electrode 14. At this time, the direction of electric field E is referenced as to x direction as described in the FIG. 1, it has a selected degree of angle with the rubbing axis.
Afterwards, before the electric field is not generated, the long axes of the liquid crystal molecules are arranged parallel to the substrate surface and parallel to the rubbing direction R. Therefore the light passed through the first polarizing plate and the liquid crystal layer is unable to pass the second polarizing plate, the screen has dark state.
When the electric field is generated, the long axes(or optical axes) are rearranged parallel to the electric field, and therefore the incident light passed through the first polarizing plate and the liquid crystal layer passes the second polarizing plate and the screen has white state.
At that time, the direction of the long axes of the liquid crystal molecules changes according to the presence of the electric field, and the liquid crystal molecules are arranged parallel to the substrate surface. Accordingly, a viewer can see the long axes of liquid crystal molecules at all points in the screen, and the viewing angle characteristic is improved.
However, the IPS mode liquid crystal display as described above also includes following problems.
As well known, the refractive anisotropy(or birefringence, n) is occurred due to the difference in the lengths of the long and the short axes of the liquid crystal molecules. The refractive anisotropy n is also varied from the viewer""s viewing directions. Therefore a predetermined color is appeared on the region where the polar angle is of 0 degree and the azimuth angle range of degrees 0, 90, 180 and 270 in spite of the white state. This regards as color shift and more detailed description thereof is attached with reference to the equation 1.
T≈T0 sin2(2"khgr")xc2x7sin2 (xcfx80xc2x7nd/xcex)xe2x80x83xe2x80x83equation 1
wherein,
T: transmittance;
To: transmittance to the reference light;
"khgr": angle between an optical axis of liquid crystal molecule and a polarizing axis of the polarizing plate;
: birefringence;
d: distance or gap between the upper and lower substrates(thickness of the liquid crystal layer); and
xcex: wavelength of the incident light.
So as to obtain the maximum transmittance T, the "khgr"should be xcfx80/4 or the nd/xcex should be xcfx80/2 according to the equation 1. As the nd varies with the birefringence difference of the liquid crystal molecules depending on viewing directions, the value of xcex is varied in order to make nd/xcex to be xcfx80/2. According to this condition, the color corresponding to the varied wavelength xcex appears.
Accordingly, as the value of n relatively decreases at the viewing directions xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d toward the short axes of the liquid crystal molecules, the wavelength of the incident light for obtaining the maximum transmittance relatively decreases. Consequently a blue color having shorter wavelength than a white color can be shown.
On the other hand, as the value of n relatively increases at the viewing directions xe2x80x9cbxe2x80x9d and xe2x80x9cdxe2x80x9d toward the short axes of the liquid crystal molecules, the wavelength of incident light relatively increases. Consequently a yellow color having a longer wavelength than the white color can be shown.
Furthermore, although the IPS-LCD is able to realize a wide viewing angle, response time thereof is very slow since long axes of the liquid crystal molecules are arranged and driven in parallel with the surface of the substrate, and no electrode is arranged at the upper substrate.
Accordingly, the object of the present invention is to provide a liquid crystal display preventing color shift generation and capable of improving display quality.
Further object of the present invention is to provide a liquid crystal display capable of improving response time characteristics.
To accomplish foregoing objects, the present invention provides a liquid crystal display comprising: a lower substrate having a plurality of gate bus lines disposed parallel to each other, a plurality of data bus lines disposed perpendicular to the gate bus lines and defining matrix type sub pixels together with the gate bus lines, a thin film transistor provided adjacent to an intersection of the gate bus line and the data bus line, and a pixel electrode connected to the thin film transistor and disposed within the sub pixel; an upper substrate opposed to the lower substrate with a selected distance and having a counter electrode, the counter electrode formed at a portion corresponding to the sub pixel and forming an electric field together with the pixel electrode; a liquid crystal layer sandwiched between and having a plurality of liquid crystal molecules; a first alignment layer and a second alignment layer formed at inner face of the lower substrate and at inner face of the upper substrate respectively; and a first polarizing plate and a second polarizing plate attached at outer face of the lower substrate and at outer face of the upper substrate respectively, wherein the electric field formed between the counter electrode and the pixel electrode is formed as an oblique line with respect to the lower substrate surface, and is formed as a diagonal line having a symmetry with respect to the data bus line and the gate bus line.
The present invention further provides a liquid crystal display comprising: a lower substrate having a plurality of gate bus lines disposed parallel to each other, a plurality of data bus lines disposed perpendicular to the gate bus lines and defining matrix type sub pixels together with the gate bus lines, a thin film transistor provided adjacent to an intersection of the gate bus line and the data bus line, and a pixel electrode connected to the thin film transistor and disposed within the sub pixel; an upper substrate opposed to the lower substrate with a selected distance and having a counter electrode, the counter electrode formed at a portion corresponding to the sub pixel and forming an electric field together with the pixel electrode; a liquid crystal layer sandwiched between and having a plurality of liquid crystal molecules; a first alignment layer and a second alignment layer formed at inner face of the lower substrate and at inner face of the upper substrate respectively; and a first polarizing plate and a second polarizing plate attached at outer face of the lower substrate and at outer face of the upper substrate respectively, wherein the counter electrode comprises a first electrode of a rectangular frame shape, and at least a second electrode disposed parallel with the gate bus line dividing a space surrounded by the first electrode into a plurality of square aperture regions; wherein the pixel electrode comprises a first branch parallel with the data bus lines and at least one or more second branches perpendicular to the first branch, and wherein an intersection of the first and the second branches is disposed at the right center of a space surrounded by the first and the second electrodes.
Furthermore, the present invention provides a liquid crystal display comprising: a lower substrate having a plurality of gate bus lines disposed parallel to each other, a plurality of data bus lines disposed perpendicular to the gate bus lines and defining matrix type sub pixels together with the gate bus lines, a thin film transistor provided adjacent to an intersection of the gate bus line and the data bus line, and a pixel electrode connected to the thin film transistor and disposed within the sub pixel; an upper substrate opposed to the lower substrate with a selected distance and having a counter electrode, the counter electrode formed at a portion corresponding to the sub pixel and forming an electric field together with the pixel electrode; a liquid crystal layer sandwiched between and having a plurality of liquid crystal molecules; a first homeotropic alignment layer and a second homeotropic alignment layer formed at inner face of the lower substrate and at inner face of the upper substrate respectively; a first polarizing plate and a second polarizing plate attached at outer face of the lower substrate and at outer face of the upper substrate respectively; and a phase compensation plate sandwiched between the second polarizing plate and the upper substrate, and having negative birefringence index, wherein the counter electrode comprises a first electrode of a rectangular frame shape, and at least a second electrode disposed parallel with the gate bus line dividing a space surrounded by the first electrode into a plurality of square aperture regions; wherein the pixel electrode comprises a first branch parallel with the data bus lines and at least one or more second branches perpendicular to the first branch, and wherein an intersection of the first and the second branches is disposed at the right center of a space surrounded by the first and the second electrodes.